1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser device (VCSEL).
2. Description of the Related Art
The vertical cavity surface emitting laser (VCSEL) device emits laser light in the direction normal to the substrate surface, and has an advantage in that a large number of such laser devices can be integrated to form a two-dimensional array on a single substrate. Thus, the VCSEL device is expected for use in a parallel optical information processing and large-capacity parallel optical transmission.
Among other VCSEL devices, a GaAs-based VCSEL device having distributed Bragg reflectors (DBRs) attracts large attention as a light source for optical communication equipment for use in the field of data communication. The GaAs-based VCSEL device includes a GaAs substrate, a pair of DBRs overlying the GaAs substrate and including a plurality of pairs of AlGaAs/AlGaAs layers having different Al contents, and at least one GaAs active layer sandwiched between the pair of AlGaAs layers as an emission area.
It is known that a VCSEL device formed on an n-type substrate has a disadvantage compared to a VCSEL device formed on a p-type substrate. The reason is as follows. A p-conductivity-type (p-type) DBR has an electric resistance inherently larger than the electric resistance of an n-type DBR. This fact necessitates a higher voltage to be applied for obtaining a necessary current between electrodes of the VCSEL device formed on the n-type GaAs substrate compared to the VCSEL device formed on the p-type GaAs substrate, in addition to the fact that the area of the p-type DBR is inherently smaller compared to the area of the n-type DBR in the VCSEL device formed on the n-type GaAs substrate due to the structure of the VCSEL device itself.
Accordingly, the GaAs-based VCSEL device developed heretofore generally has a p-type GaAs substrate, a p-type DBR formed on the substantially entire surface of the p-type GaAs substrate, an active layer and n-type DBR which are formed as an air post structure on the p-type DBR, wherein the electric resistance of the p-type DBR is reduced.
The DBR in the developed GaAs-based VCSEL device includes at least one AlGaAs layer having a highest Al content among the semiconductor layers in the VCSEL device. A specified area of the at least one AlGaAs layer in the DBR is selectively oxidized to form an Al-oxidized area having a higher electric resistance and which acts as a current confinement structure, the injected current path being limited to outside the Al-oxidized area. This structure achieves excellent lasing characteristics; for example, a higher emission efficiency is obtained with a low threshold current.
Nevertheless, there is a problem in that the thermal saturation characteristic of the optical output power in the conventional VCSEL device having a wavelength of 850 nanometers (nm) is not satisfactory. More specifically, in a high ambient temperature, the maximum optical output power of the VCSEL device is saturated and lower than the desired optical output power. This effect is shown in FIG. 3, which depicts the onset tendency of saturation appearing above about 50xc2x0 C., and saturation as occurring at a temperature of about 70xc2x0 C. and at an optical output power of 8.5 milli-watt (mW), whereby the optical output power cannot be increased further irrespective of the intensity of the injected current. The saturation problem is not limited to the above example of a lasing wavelength of 850 nm, and is common to other VCSEL devices irrespective of the lasing wavelength thereof.
In one aspect of the invention, a semiconductor laser comprises an active layer, a selectively oxidized layer forming a current confinement structure for channeling current through the active layer, and first and second distributed Bragg reflectors (DBR) sandwiching the active layer and the selectively oxidized layer. Each DBR comprises layers of material having different refractive indices. In addition, the first distributed Bragg reflector comprises two portions, a lower thermal conductivity portion and a higher thermal conductivity portion. Preferably, the higher thermal conductivity portion comprises layers of a first material having a thermal conductivity of at least about 50 W/Km.
In another aspect of the invention, a vertical cavity surface emitting laser (VCSEL) device comprises a substrate, a bottom distributed Bragg reflector (DBR) disposed over the substrate, a selectively oxidized layer formed over the bottom DBR, at least one active layer formed over the selectively oxidized layer, and a top distributed Bragg reflector (DBR) formed over the active layer. The bottom DBR is divided into upper and lower sections, each comprising a plurality of layers of semiconductor material. The lower section is proximate to the substrate and includes one or more aluminum containing and substantially gallium free layers. All of the plurality of layers forming the upper section include both aluminum and gallium. The selectively oxidized layer over the bottom DBR forms a current confinement structure.
In still another aspect of the invention a vertical cavity surface emitting laser (VCSEL) device includes a substrate, bottom and top distributed Bragg reflectors (DBRs) overlying the substrate, each of the bottom and top DBRs including a plurality of first semiconductor layers and a plurality of second semiconductor layers each disposed for a corresponding one of the first semiconductor layers to form a pair and having a refractive index lower than a refractive index of the first semiconductor layers, at least one active layer sandwiched between the bottom DBR and the top DBR, the bottom DBR including AlAs layers as the second semiconductor layers, one of the AlAs layers being formed as a selectively oxidized layer having therein an Al-oxidized area for defining a current confinement structure, and at least one anti-oxidation layer disposed between the one of the AlAs layers and another of the AlAs layers, the anti-oxidation layer having an oxidation rate lower than an oxidation rate of the AlAs layers.
In accordance with this VCSEL device, the anti-oxidation layer between the selectively oxidized AlAs layer formed as the top layer of the bottom DBR and the AlAs layer of the bottom DBR effectively prevents the AlAs layers in the bottom DBR from being oxidized. The thermal resistance of the bottom DBR can thereby be reduced and thus an excellent thermal saturation characteristic of the VCSEL device can be obtained. Consequently, this device is capable of operating at a higher output power with a higher stability in a high ambient temperature environment.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.